Signal transmission fuse material has become widely used in mining and quarrying operations, as well as in the military, for non-electric blasting of explosive charges from a remote location. The 1971 U.S. Pat. No. 3,590,739 to Persson shows a plastic tube with an outer diameter of approximately 3 mm and an inner diameter of slightly over 1 mm such that the ratio of the inner to the outer circumference of the tube is approximately 0.35 or less. The interior bore of the tube is coated with a reactive material. The reactive powder material has evolved since the date of the Persson patent and the most common coating material now used is a mixture of HMX and aluminum powder as disclosed in the U.S. Pat. No. 4,328,753.
The U.S. Pat. No. 4,328,753 patent issued in 1982 discloses a low energy fuse in the form of a plastic tube having concentric tubular plies, the inner ply comprising SURLYN (Dupont), to which material the reactive powder material can be conveniently coated. The outer tube or ply is preferably fabricated from a mechanically tougher material having greater tensile strength and abrasive resistance, as well as being less expensive. Thus, the outer material may comprise a polyamide or polypropylene or polyolefin or similar polymer designed to withstand the stress encountered at a worksite. This prior art U.S. Pat. No. 4,328,753 suggests that the outer diameter be 3 mm, and the inner diameter be 1.3 mm, and that the core loading be 2.7 grams of reactive material per square meter of inside surface area.
U.S. Pat. No. 5,212,341 suggests a coextruded triple plastic layer shock tube designed with the intermediate layer providing better bonding between the inner and outer plies. Finally, U.S. Pat. 5,597,973 suggests reducing the outer diameter of the polymeric shock tube to approximately 2.15 mm, and the wall thickness being approximately 0.7 mm or less. The resulting shock tube has about 2.7 grams per square meter reactive powder loading from the 1982 U.S. Pat. No. 4,328,753 mentioned previously.
All of these prior art patents suggest that any practical shock tube comprises an annular cross section such that the inner diameter of the shock tube is dictated by the reactive powder loading in g/m2 and the outer diameter is dictated by the wall thickness required to contain the percussive pressures created inside the tube.